Devil Arm

Devil Arms (魔具) are demonic, variably sentient weapons that are commonly the manifestation of a defeated Demon's soul or power. In the Devil May Cry series, Devil Arms are considered to be the characters' most powerful weapons. When wielded, Devil Arms often convey their power to the wielder, provided they have been subdued first.

Description
The exact process by which a Devil Arm is created is never revealed. However, in Devil May Cry 3, Dante receives new Devil Arms by defeating strong demons and their souls manifesting as weapons, although in case with the Sword of Sparda, it might be possible that a demon doesn't have to die to manifest its power as a Devil Arm.

The process of subduing a Devil Arm seems to differ from game to game. In Devil May Cry 3 Dante simply had to kill a boss for it to agree to "help" him in recognition of his power. Their souls then became Devil Arms. In Devil May Cry the weapon is found by Dante and the process of subduing it requires Dante to survive the weapons' attempt to kill him.

Although Devil Arms are primarily used as weapons, they are also shown to be used for other purposes. The Yamato was used by the Dark Knight Sparda to seal the true Hell Gate, and Agnus used several other Devil Arms to power his imitations of that Hell Gate.

Sparda also sealed the power of his sword by using the Perfect Amulet.

Devil May Cry 3
At the start, Dante wields only Rebellion, the keepsake from his father. He eventually acquires new Devil Arms by defeating the demons guarding the Temen-ni-gru: Cerberus, Agni & Rudra, and Nevan. Vergil, on the other hand, gets only two other Devil Arms aside from Yamato: Beowulf, and Force Edge. After the 2nd boss battle against Vergil, Vergil loses Beowulf and Dante acquires it for his own use.

Each Devil Arm not only grants the twins new powers and abilities, but also seems to exert some influence over their Devil Triggers; Dante's physical shape in Devil Trigger form changes depending on the Devil Arm he wields, while Vergil's form does not change, but the scabbard of his Yamato changes to a green, scaly carapace reminiscient of the scales that cover Vergil's coat and body, and equipping Beowulf covers his chest in a glowing cuirass that somewhat resembles a ribcage.

Although Artemis is referred to as "a gun of the demon world", it is also unknown whether it is also a Devil Arm.

Devil May Cry
The Force Edge is used by Dante as his default weapon in the first game. An alternative Devil Arm, the Alastor, can also be found, with the capability of giving Dante new demonic powers. In order to wield the Alastor, Dante had to first prove himself to the spirit of the blade. He does this by surviving being impaled through the chest by Alastor. Yet another Devil Arm, the Ifrit, is also discovered. It lifts Dante off the ground by his arms and tries to pull him in two directions at once, but Dante is too strong for it to succeed, and it submits to him. Dante also recovers both halves of the Perfect Amulet and re-awakens the Force Edge, turning it back into Sparda.

While on Mallet Island, the Devil Arms Dante uncovers also alter his Devil Trigger form, just like the ones from the Temen-ni-gru.

Devil May Cry 4
Before the start of the game, the Order of the Sword collected several Devil Arms (Gilgamesh, Lucifer and Pandora) and used them to power several Hell Gates and keep them open. Only a few strong demons are able to use these Hell Gates to enter the human world, and when they are defeated, they do not relinquish their souls as Devil Arms. Instead, Dante takes possession of separate, already quiescent Devil Arms being used to hold the gates open.

Devil Arms can be broken, but fixing them is depicted as a feat of incredible power. Agnus was unable to restore Yamato no matter what he did, despite being an expert alchemist, but the sword is easily fixed by Nero when he awakens his Devil Trigger.

Devil May Cry 2
Dante only uses one Devil Arm on his journey to Dumary Island, his trusty Rebellion.

DmC: Devil May Cry
has only one melee weapon, the, but it is capable of shapeshifting into multiple angelic and demonic weapons: the Ophion, Arbiter, Osiris, Eryx, and Aquila. The Arbiter and Osiris forms are awakened when Dante investigates the portraits of his father and mother while in the Limbo-version of his family's old mansion, while the Eryx and Aquila forms are awakened after Dante slays the demon lords Poison and Bob Barbas.